Not applicable.
1. Field of the Invention
The present invention relates to a liquid crystal display device, and more specifically to a liquid crystal display having an alignment film that reduces image sticking.
2. Description of Prior Art
Flat panel displays have been become increasingly important in the computer industry and in other industries requiring the display of information. Such displays provide unique opportunities for lowering the weight, size, and eventually cost of displaying information. Presently, liquid crystal display (xe2x80x9cLCDxe2x80x9d) devices appear to hold the most promise as the technology that will eventually be used for nearly all flat panel displays. Considerable success has been achieved in using liquid crystal technology for displaying information in small size color televisions, laptop computers, and projection systems, and LCDs are increasingly being used for desktop computers.
The desirability of LCDs has produced an international industry having several billion dollars in annual sales. One conventional LCD device includes liquid crystal cells having liquid crystal sandwiched between transparent electrodes formed on opposed glass substrates (i.e., plates). Another conventional LCD device includes liquid crystal cells having liquid crystal sandwiched between a transparent electrode on glass substrate and a reflective electrode on a non-transparent substrate (e.g., silicon). In either case, an electrical signal is selectively applied between the electrodes to allow the device to modulate incident light and display information.
To obtain contrast, the orientation of the liquid crystal molecules must be uniformly controlled. In a field effect system such as a nematic system (which is designed to twist the liquid crystal molecules by about 0xc2x0 to 270xc2x0 between the upper and lower plates), it is preferable to orient the liquid crystal molecules parallel to the substrate surface in a unidirectional manner. The liquid crystals can be aligned through stretching a polymer, rubbing a polymer, depositing a polymer in the form of a Langmuir Blodgett film, or exposing a polymer film to UV radiation. Additionally, alignment can be achieved by depositing particles of SiO on substrates or by etching grooves using microlithography.
One popular alignment technique is to deposit a polyimide alignment film on each transparent electrode to orient the liquid crystal, and then to rub or abrade the polyimide film in a desired direction. More specifically, a polyimide film is formed by applying a wet coat of polyimide to a substrate using known printing or spinning techniques. The wet coat is baked to form a polyimide film on the substrate. After the polyimide film is formed on the substrate, the atomic structure of the film must be aligned in a desired direction in order to orient the liquid crystal molecules in the desired direction. For this purpose, the polyimide film is rubbed in the desired direction with a gigged, flocked, or velvet cloth, and then cleaned to remove debris from the rubbing. In this manner, an alignment film is formed as an insulating layer with an atomic structure aligned so as to orient the liquid crystal molecules in the desired direction.
The LCD device is formed by sandwiching a thin (e.g., five micron) layer of liquid crystal between two glass substrates having transparent conductors and other thin layers of materials that provide electronics and optical filters. When a voltage is applied across the thin layer of liquid crystal, the liquid crystal molecules respond by rotating to minimize the electrostatic energy of the system. This behavior is used to form a light switch that is turned on and off by controlling the rotation of the liquid crystal molecules using an external voltage. A large, addressable array of such liquid crystal light switches is used in the LCD device.
While each liquid crystal light switch should switch on and off instantaneously with the switching of the applied voltage, the thin layer of material used for aligning the liquid crystal in conventional LCD devices accumulates a charge that only leaks slowly over time. For example, when the above-described process is used to align the polyimide film, the film is altered in such a way that the application of a voltage causes charge redistribution to occur. This effect decays relatively slowly over time after the removal of the voltage.
When charge accumulates on a portion of the alignment film, nearby liquid crystal molecules are exposed to a residual voltage after the applied voltage is switched off. The slow decay of this residual voltage results in a slow change in the alignment of the liquid crystal molecules. Thus, with a large array of liquid crystal switches, the LCD device can develop regions with residual charge that keeps the liquid crystal light switches switched locally. In such regions, the displayed image is retained after the applied voltage is switched off. This is known as image sticking and is a highly undesirable drawback of conventional LCD devices. While image sticking can be reduced by carefully choosing the type of alignment layer and the alignment technique, this prevents optimization both in terms of display performance and manufacturing cost.
In view of these drawbacks, it is an object of the present invention to remove the above-mentioned drawbacks and to provide a liquid crystal display device in which image sticking is significantly reduced or eliminated. The LCD device is formed with a very thin alignment film that allows for charge hopping or tunneling. This significantly reduces charge accumulation so that image sticking is greatly minimized or eliminated. Thus, the LCD device can provide a very high quality display.
A first embodiment of the present invention provides a liquid crystal display device having first and second substrates. A first electrode layer overlies one surface of the first substrate, and a second electrode layer overlies one surface of the second substrate. A first alignment layer having a thickness of 100 xc3x85 or less overlies the first electrode layer. Additionally, a second alignment layer overlies the second electrode layer, and a liquid crystal material is disposed between the first and second alignment layers. In one preferred embodiment, the second alignment layer also has a thickness of 100 xc3x85 or less, and both alignment layers are polyimide layers.
A second embodiment of the present invention provides a method for manufacturing a liquid crystal display device that has reduced image sticking. According to the method, first and second substrates are provided, a first electrode layer is deposited over one surface of the first substrate, and a second electrode layer is deposited over one surface of the second substrate. A first alignment layer having a thickness of 100 xc3x85 or less is deposited over the first electrode layer, and a second alignment layer is deposited over the second electrode layer. Additionally, the first and second substrates are arranged so that the first and second alignment layers face one another and a space is formed between the substrates. A liquid crystal material is disposed in the space between the first and second substrates. In a preferred method, the first alignment layer is deposited by diluting polyimide with a solvent, and spinning, spraying, or printing the diluted polyimide on the first substrate. The atomic structure of the applied polyimide is preferably aligned using ion beam bombardment.
Other objects, features, and advantages of the present invention will become apparent from the following detailed description. It should be understood, however, that the detailed description and specific examples, while indicating preferred embodiments of the present invention, are given by way of illustration only and various modifications may naturally be performed without deviating from the present invention.